COMPARISON OF METHODS TO PREVENT RESTRICTOR PLUGGING DURING OFF-LINE SUPERCRITICAL EXTRACTION

Real-world environmental samples which contain high concentrations of water and/or extractable matrix components frequently cause intermittent or irreversible plugging of capillary flow restrictors during off-line supercritical fluid extraction. Heating the entire restrictor at 50-degrees-C produces a constant extraction flow-rate (+/-0.03 ml/min) for such samples, but poor collection efficiencies (30-65% recovery for polycyclic aromatic hydrocarbons) were obtained because the supercritical fluid could not be depressurized directly into the collection solvent. While the collection efficiency was improved (80-90%) by nebulizing an organic solvent with the restrictor effluent, a simpler method was to heat all but the last 3 cm of the restrictor and to depressurize the extract directly in the collection solvent. Depending on the sample matrix, restrictor heater temperatures ranging from 50 to 200-degrees-C were required to avoid restrictor plugging. With proper heating, constant extraction flow-rates (+/-0.07 ml/min) and high collection efficiencies (90-100%) for polycyclic aromatic hydrocarbons and n-alkanes as volatile as n-octane were achieved.